This invention relates to a novel method of preparing a silver catalyst and its use in the process of making ethylene oxide by the partial oxidation of ethylene in the vapor phase. Silver-containing catalysts in which the catalytically active component is the metal itself are well known in the art. An important use for the catalyst is in the direct oxidation conversion of alkenes to the corresponding vicinal epoxides, particularly in preparing ethylene oxide from ethylene by reacting ethylene with oxygen in the vapor phase.
Methods known to the art for making such catalysts include soaking a carrier or support in aqueous solutions of silver salts to impregnate it. Thereafter the thus-impregnated salts are reduced to silver metal prior to utilization in the process for oxidizing ethylene. Reduction is normally accomplished by heating in the presence of a reducing agent or by thermal decomposition of the salt. This is done at temperatures within the range of 125.degree. C. to 400.degree. C. and preferably from 200.degree. C. to 300.degree. C. Alternatively, the silver salt may be deposited from a slurry. Either slurry or solution also may contain a reducing agent, or the reducing agent may be subsequently applied.
The commonly used reducing agents are organic compounds which include polyhydric alcohols, such as liquid glycols (e.g. ethylene, propylene, and butylene glycols), glycerol, aqueous sugar solutions, aqueous polyvinyl alcohol solutions, the polyglycols, (e.g. polyethylene and polypropylene glycols) preferably of relatively low molecular weight; also included are aqueous solutions of such polyglycols, the water soluble glycol alkyl ethers, and the like. Other excellent reducing agents are high-boiling esters of carboxylic acids such as diethyl sebacate, dibutyl sebacate, dioctyl sebacate, dicapryl sebacate, diethyl phthalate dibutyl phthalate, dibutyl azelate, dioctyl azelate and dicapryl azelate.
One of the criteria for commercially useful silver catalysts is that the silver be finely divided and relatively homogeneously dispersed on the catalyst support. Dispersing agents are advantageously used in order to obtain such silver deposits, especially suitable as dispersing agents are organic amines such as ethylene diamine and ethanolamine and others disclosed in U.S. Pat. No. 3,702,259; and those naturally occurring gums such as disclosed in U.S. Pat. No. 3,887,491. These natural gums are, for example, karaya, ghatti, and tragacanth, which are plant exudates; root or seed extracts, such as guar, saponin and locust bean, psyllium seed, and quince seed. Seaweed extracts such as agar, carrageenin and furcellaran are also useful as well as others such as gelatin, casein, and pectin. Certain chemically modified derivatives of starch, of cellulose and poly sacharides (the unmodified forms of which are insoluble) are also included as substances classifiable as gums and are useful as dispersing agents in the preparation of catalysts.
While aqueous solutions are usually satisfactory to use in dissolving the dispersing agents, those which are less water soluble may be soluble in one of the aliphatic alcohols having from 1 to 4 carbon atoms, or may be soluble in mixtures of alcohol with water. Mixtures of an alcohol with water are particularly useful if the silver salt which is being employed is not sufficiently soluble in the alcohol alone. Representative silver salts which may be employed are silver salts of certain inorganic acids for example, silver nitrate, silver chlorate, and silver metaborate, or salts of carboxylic acids such as silver acetate, silver propionate and silver formate may be used. The preferred salt is silver nitrate because it is so readily soluble and easily reduced, either thermally or with an organic reducing agent or hydrogen.
Supports known to be useful for making silver catalysts are for example alumina, zirconia, corundum, mullite, silicon carbide and carbon. Alumina is preferred and especially a porous alumina of low surface area, i.e. less than one square meter per gram.
While silver is the metal most useful from a commercial standpoint in providing the catalytic effect necessary to obtain ethylene oxide, most commercial catalysts additionally contain small amounts of a promoter. The amount employed is usually from a few parts per million up to one or two percent, based on the weight of the total catalyst. Representative promoters include the alkali and alkaline earth metals which are usually present as their oxides. Thus lithium, sodium, potassium, rubidium, cesium, calcium, barium, cadmium, and the like, are added as their salts to the solution of the silver salt which is applied to the support and on subsequent heating are converted to their oxides.
Other ways known to the art of adding the promoter compound are to add it to the support prior to or subsequent to the application of the silver salt. In each case the particular salt applied is dried prior to applying the solution of the second salt. Generally the promoters, when applied first, are converted to their oxides and the silver salt when applied first, is reduced to silver. To insure adequate penetration of the pores of the support, a vacuum is applied when applying the aqueous solutions of the silver salt or of the promoter salt. This is described in U.S. Pat. No. 3,575,888.
An example of a procedure for preparing a good catalyst of the prior art is to soak an alumina support in an excess of an aqueous solution containing the silver nitrate and barium nitrate as a promoter along with a dispersing agent, such as saponin, soaking for a period of time and following that with a period of draining in order to remove excess liquid. The wetted support is then dried at atmospheric pressure, or under reduced pressure, or under a flow of nitrogen for a period of time sufficient to dry the support. The burdened, dried support is then reduced by immersing it in a bath of high-boiling mineral oil containing a small amount of a reducing agent such as the high-boiling esters previously mentioned. Although a high-boiling ester can be used neat, it is preferred to use the ester at a rate of about 10% in the mineral oil for economic considerations. Either way, once the reduction is complete the excess oil and or high-boiling ester must be drained off and any remaining on the catalyst support needs to be vaporized or burned off by heating.
According to the present invention, using a modification of the previously described process which is representative of a good process known in the prior art a catalyst has been prepared which gives excellent conversions equivalent to those known to the prior art, and better yields, but does so at a process temperature which is 5 to 10 degrees lower than normally expected to provide a saving in energy and potentially a longer useful life.